This invention relates to medical implants formed of a polymeric material such as ultra-high molecular weight polyethylene, with superior oxidation and wear resistance produced by a sequential irradiation and annealing process.
Various polymer systems have been used for the preparation of artificial prostheses for biomedical use, particularly orthopedic applications. Among them, ultra-high molecular weight polyethylene is widely used for articulation surfaces in artificial knee, hip, and other joint replacements. Ultra-high molecular weight polyethylene (UHMWPE) has been defined as those linear polyethylenes which have a relative viscosity of 2.3 or greater at a solution concentration of 0.05% at 135° C. in decahydronaphthalene. The nominal weight—average molecular weight is at least 400,000 and up to 10,000,000 and usually from three to six million. The manufacturing process begins with the polymer being supplied as fine powder which is consolidated into various forms, such as rods and slabs, using ram extrusion or compression molding. Afterwards, the consolidated rods or slabs are machined into the final shape of the orthopedic implant components. Alternatively, the component can be produced by compression molding of the UHMWPE resin powder.
All components must then go through a sterilization procedure prior to use, but usually after being packaged. There exists several sterilization methods which can be utilized for medical applications, such as the use of ethylene oxide, gas plasma, heat, or radiation. However, applying heat to a packaged polymeric medical product can destroy either the integrity of the packaging material (particularly the seal, which prevents bacteria from going into the package after the sterilization step) or the product itself.
It has been recognized that regardless of the radiation type, the high energy beam causes generation of free radicals in polymers during radiation. It has also been recognized that the amount or number of free radicals generated is dependent upon the radiation dose received by the polymers and that the distribution of free radicals in the polymeric implant depends upon the geometry of the component, the type of polymer, the dose rate, and the type of radiation beam. The generation of free radicals can be described by the following reaction (which uses polyolefin and gamma ray irradiation for illustration):                gamma raysPolyolefin----------------r. where r. are primary free radicals*  (1)*(through C—C chain scission or C—H scission)        
Depending on whether or not oxygen is present, primary free radicals r. will react with oxygen and the polymer according to the following reactions as described in “Radiation Effects on Polymers,” edited by Roger L. Clough and Shalaby W. Shalaby, published by American Chemical Society, Washington, D.C., 1991.
In the presence of oxygen
O2 r.-----------rO2  (2)rO2+polyolefin-------rOOH+P.  (3)P.+O0----------------P02.  (4)                O2 P02.+polyolefin----POOH+P.--------P02.  (5)rO2., P02.------Some chain scission products  (6)        room temperaturerOOH, POOH--------------free radicals, rOH, POH  (7)P.+P02------POOP (ester cross-links)  (8)2P.------------P—P (C—C cross-links)  (9)        
In radiation in air, primary free radicals r. will react with oxygen to form peroxyl free radicals r02., which then react with polyolefin (such as UHMWPE) to start the oxidative chain scission reactions (reactions 2 through 6). Through these reactions, material properties of the plastic, such as molecular weight, tensile and wear properties, are degraded.
It has been found that the hydroperoxides (rOOH and POOH) formed in reactions 3 and 5 will slowly break down as shown in reaction 7 to initiate post-radiation degradation. Reactions 8 and 9 represent termination steps of free radicals to form ester or carbon-carbon cross-links. Depending on the type of polymer, the extent of reactions 8 and 9 in relation to reactions 2 through 7 may vary. For irradiated UHMWPE, a value of 0.3 for the ratio of chain scission to cross-linking has been obtained, indicating that even though cross-linking is a dominant mechanism, a significant amount of chain scission occurs in irradiated polyethylene.
By applying radiation in an inert atmosphere, since there is no oxidant present, the primary free radicals r. or secondary free radicals P. can only react with other neighboring free radicals to form carbon-carbon cross-links, according to reactions 10 through 12 below. If all the free radicals react through reactions 10 through 12, there will be no chain scission and there will be no molecular weight degradation. Furthermore, the extent of cross-linking is increased over the original polymer prior to irradiation. On the other hand, if not all the free radicals formed are combined through reactions 10, 11 and 12, then some free radicals will remain in the plastic component.
In an Inert Atmospherer.+polyolefin-----------P.  (10)2r.-----------r-r (C—C cross-linking)  (11)2P.----------P—P (C—C cross-linking)  (12)
It is recognized that the fewer the free radicals, the better the polymer retains its physical properties over time. The greater the number of free radicals, the greater the degree of molecular weight and polymer property degradation will occur. Applicant has discovered that the extent of completion of free radical cross-linking reactions is dependent on the reaction rates and the time period given for reaction to occur.
UHMWPE is commonly used to make prosthetic joints such as artificial hip joints. In recent years, it has been found that tissue necrosis and interface osteolysis may occur in response to UHMWPE wear debris. For example, wear of acetabular cups of UHMWPE in artificial hip joints may introduce microscopic wear particles into the surrounding tissues.
Improving the wear resistance of the UHMWPE socket and, thereby, reducing the rate of production of wear debris may extend the useful life of artificial joints and permit them to be used successfully in younger patients. Consequently, numerous modifications in physical properties of UHMWPE have been proposed to improve its wear resistance.
It is known in the art that ultrahigh molecular weight polyethylene (UHMWPE) can be cross-linked by irradiation with high energy radiation, for example gamma radiation, in an inert atmosphere or vacuum. Exposure of UHMWPE to gamma irradiation induces a number of free-radical reactions in the polymer. One of these is cross-linking. This cross-linking creates a 3-dimensional network in the polymer which renders it more resistant to adhesive wear in multiple directions. The free radicals formed upon irradiation of UHMWPE can also participate in oxidation which reduces the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement and a significant increase in wear rate. The free radicals are very long-lived (greater than eight years), so that oxidation continues over a very long period of time resulting in an increase in the wear rate as a result of oxidation over the life of the implant.
Sun et al. U.S. Pat. No. 5,414,049, the teachings of which are incorporated herein by reference, broadly discloses the use of radiation to form free radicals and heat to form cross-links between the free radicals prior to oxidation.
Hyon et al. U.S. Pat. No. 6,168,626 relates to a process for forming oriented UHMWPE materials for use in artificial joints by irradiating with low doses of high-energy radiation in an inert gas or vacuum to cross-link the material to a low degree, heating the irradiated material to a temperature at which compressive deformation is possible, preferably to a temperature near the melting point or higher, and performing compressive deformation followed by cooling and solidifying the material. The oriented UHMWPE materials have improved wear resistance. Medical implants may be machined from the oriented materials or molded directly during the compressive deformation step. The anisotropic nature of the oriented materials may render them susceptible to deformation after machining into implants.
Salovey et al. U.S. Pat. No. 6,228,900, the teachings of which are incorporated by reference, relates to a method for enhancing the wear-resistance of polymers, including UHMWPE, by cross-linking them via irradiation in the melt.
Saum et al. U.S. Pat. No. 6,316,158 relates to a process for treating UHMWPE using irradiation followed by thermally treating the polyethylene at a temperature greater than 150° C. to recombine cross-links and eliminate free radicals.
Several other prior art patents attempt to provide methods which enhance UHMWPE physical properties. European Patent Application 0 177 522 81 relates to UHMWPE powders being heated and compressed into a homogeneously melted crystallized morphology with no grain memory of the UHMWPE powder particles and with enhanced modulus and strength. U.S. Pat. No. 5,037,928 relates to a prescribed heating and cooling process for preparing a UHMWPE exhibiting a combination of properties including a creep resistance of less than 1% (under exposure to a temperature of 23° C. and a relative humidity of 50% for 24 hours under a compression of 1000 psi) without sacrificing tensile and flexural properties. U.K. Patent Application GB 2 180 815 A relates to a packaging method where a medical device which is sealed in a sterile bag, after radiation/sterilization, is hermetically sealed in a wrapping member of oxygen-impermeable material together with a deoxidizing agent for prevention of post-irradiation oxidation.
U.S. Pat. No. 5,153,039 relates to a high density polyethylene article with oxygen barrier properties. U.S. Pat. No. 5,160,464 relates to a vacuum polymer irradiation process.